It is nowadays becoming possible to make images of near offset quality, especially colour images, using non-offset printing techniques, such as electrophotography. In duplex printing, such an image is made on both sides of a receptor support, such as a paper sheet.
In an electrophotographic apparatus, highest throughput in duplex printing on pre-cut sheets is obtained by making both images in a single pass through the subsequent stations in the apparatus--these stations are described in more detail below. Such single pass duplex printing requires transporting a sheet holding a toner image at both sides, since initially, after both toner images are made and transferred to the sheet, the images are adhering only loosely to the sheet. They can easily be removed, e.g. by rubbing or by a slight contact. Only in the next processing step, in the fusing station, the images are permanently fixed onto the sheet.
The problem of transporting a sheet holding an unfixed double-sided toner image also exists, when this image is obtained using other electrographic systems. In direct electrostatic printing, for example switchable aperture electrodes image-wise regulate the toner transfer through the printhead to the sheet. Also in this case, single pass processing of duplex images requires transporting a receptor support holding a double-sided unfixed toner image to a fusing station.
U.S. Pat. No. 4,427,285 describes a single pass duplex reproduction system having a heat insulating prefuser transport device. In particular, the prefuser transport is a pair of cold, toner compacting rolls.
A drawback of the system disclosed there is that it requires a film of release agent to be deposited onto the compacting rolls, to prevent toner offsetting onto these rolls.
A restriction of such a system is that the speed of the transfer stations, located before the prefuser transport device, and the speed of the fuser station, located after the prefuser transport device, must be substantially equal.
Research Disclosure No. 16249 of December 1977 shows a dielectric belt for transporting copy sheets having unfused toner images on one or both sides. The belt is supported by a first and a second roller, and transports the copy sheet while it is suspended to the belt. The belt is charged by an electrostatic charger, causing an electrostatic attraction force, which tacks the copy sheet to the belt. A neutralising charger, positioned near the second roller, sprays neutralising charges onto the belt. The copy sheet is separated from the belt through the combined effects of neutralisation of the electrostatic attractive force by the neutralising charger, and the sharp bending of the belt about the second roller.
A drawback of this system is that the electrostatic attraction between the belt on the one hand, and the copy sheet containing the toner image on the other hand, may result in toner being transferred from the copy sheet to the belt, thus causing damage to the toner image contacting the belt.
U.S. Pat. No. 5,009,352 describes a conveyor for transporting sheets having an unfused toner image on one side. The conveyor is charged by an electrostatic charger. The conveyor comprises a dielectric material and electrode parts of a conductive material, in order to create a strong and stable electrostatic field. The electrostatic field causes the sheets to adhere to the conveyor. The conveyor may e.g. be a belt. The conveyor contacts the side of the sheet that does not carry the toner image.
This system is not intended for conveying sheets having unfused toner images on both sides. However, for transporting sheets having unfused toner images on both sides, or for transporting sheets having only one toner image that contacts the conveyor, this system has the same drawback as the previous system--shown in Research Disclosure No. 16249--in that the electrostatic attraction may cause damage to the toner image contacting the conveyor.
U.S. Pat. No. 5,045,892 describes an arrangement to convey paper, having an unfused toner image on one side, from a transfer station to a fusing station. The arrangement comprises a guide member to convey the paper under the control of an electrostatic force. Some of the shown embodiments comprise an endless belt; in the embodiments comprising a belt, the paper is conveyed suspended to the belt, so that the unfused toner image does not contact the belt. The belt is charged by an electrostatic charger so as to attract the paper. At least in the second embodiment, the toner image is attracted to the outer surface of the belt. In some embodiments, the paper may be charged by a second electrostatic charger with an opposite polarity to the belt.
This system is not intended for conveying paper having unfused toner images on both sides, just as the previous system is not. The drawback mentioned above applies to the three last systems described above--i.e. to the systems shown in Research Disclosure No. 16249, in U.S. Pat. No. 5,009,352 and in U.S. Pat. No. 5,045,892--viz. the drawback that the electrostatic attraction may cause damage to a toner image contacting the conveying belt. This is especially important if images of high, near offset quality are to be obtained, as in the present invention.